The present invention relates generally to a plant propagation system, and more particularly to an automated system and process for promoting heterotrophic growth of plant tissue.
Micropropagation, sometimes referred to as tissue culture propagation, is the process of growing new plants from a piece of plant tissue that has been extracted from a parent plant with desired characteristics. Micropropagation has recently grown in popularity as a preferred plant propagation technique for a wide range of horticultural crops because of high production efficiency and greater uniformity of the resulting plants. The process results in the mass reproduction of plants having certain desirable characteristics since substantially all of the plants produced are genetically identical to and have all of the desirable traits of the parent. Micropropagation is an especially useful process for genetically engineered plants, high-value transplants, seedless fruits and vegetables, certified disease free plant material and all other plants that cannot be produced from seed economically or uniformly.
In general terms, micropropagation typically includes first selecting a parent plant. The parent plant should be healthy and should have the desired traits that are needed in the next generation plants. A tissue sample is then extracted from the parent. The sample is typically meristematic tissue which is undifferentiated tissue capable of dividing and giving rise to other meristemic tissue as well as specialized tissue types. Meristematic tissue is found in growth areas such as at the tips of stems or at lateral buds. The tissue sample (explant) is disinfected and then placed in a controlled environment and supplied essential nutrients for promoting growth.
Growth of the plant tissue sample into a small plant occurs in four commonly referred to stages. First, growth of the explant is established in a sterile environment. Second, high proliferation of explant is promoted by repeated selection of small pieces of tissue containing vegetative buds, or other specialized propagative structures (e.g., bulbets, protocorm-like bodies (PLB), microtubers, somatic embryos). The third stage involves forming a shoot from the vegetative bud. The fourth stage involves forming a root on the shoot, thereby completing the development of a whole plant from the plant tissue.
During the first and second stages of growth, the plant tissue is made up of small rapidly dividing cells with high metabolic requirements for energy. The tissue is incapable of carrying out adequate photosynthesis to meet this high demand.
Consequently, initial growth of the tissue is done heterotrophically. Heterotrophic growth is where the organism obtains nourishment and energy from the ingestion and breakdown of organic matter. During this phase, the plant tissue is typically not exposed to light and is fed a growth medium containing organic carbon. The organic carbon is usually obtained from sugars such as sucrose.
In the third stage of growth, leaves and shoots expand and the plant tissue becomes more capable of photosynthesizing. The plant tissue, when exposed to light, gases, water and essential nutrients, derives energy photoautotrophically through the process of photosynthesis. Photoautotrophic growth is where an organism synthesizes organic nutrients by deriving energy from light. In other words, during autotrophic growth, the plant tissue is capable of making its own food which it cannot do adequately during the other stages.
The focus of the process and system of the present invention is the initial heterotrophic growth of plant tissue. Generally, this stage of growth involves placing explant tissue in contact with a nutrient medium formulated to provide everything to which the tissue would have access if it were part of a complete plant. Hormones can also be added to the nutrient growth media in order to stimulate desired growth responses.
Traditionally, the heterotrophic growth of plant tissue has been done in a batch type arrangement. Specifically, tissue samples have been placed on agar or semisolid mediums for providing nutrients and organic carbon to the plant material. Once the nutrient medium is spent, the plant tissue is manually transplanted to new media for continued growth. However, this process is not only expensive and time consuming but can lead to contamination of the plant material since aseptic conditions are almost impossible to maintain.
Recently, attempts have been made to develop a plant micropropagation system that does not rely on semisolid mediums, using instead a liquid nutrient solution. Examples of plant growth systems are illustrated in U.S. Pat. No. 3,578,431 to Ingestad, et al., U.S. Pat. No. 4,320,594 to Raymond, U.S. Pat. No. 4,669,217 to Frase, U.S. Pat. No. 4,934,096 to Bentvelsen, U.S. Pat. No. 5,049,505 to Sei, U.S. Pat. No. 5,184,420 to Papadopoulos, et al., U.S. Pat. No. 5,104,527 to Clinkenbeard, U.S. Pat. No. 5,139,956 to Schick, et al., and U.S. Pat. No. 5,186,895 to Onofusa, et al. However, as will be apparent to one skilled in the art the particular features and aspects of the present invention remain absent from the prior art.